This invention relates generally to carbide lime used as a filler in thermoplastic compositions, and more particularly to a synthetic carbide lime filler and method of making the same.
Canadian Patent Nos. 2,213,086 and 2,296,609, and U.S. Pat. No. 6,310,129 (“the '129 patent”), which are hereby incorporated herein by reference, each patent describe a method for converting raw “carbide lime” waste material into a composition usable as a filler in thermoplastic resin materials. The raw carbide lime material is the waste by-product of acetylene gas production. All three patents contain essentially the same disclosure, and the '129 patent will typically be referred to hereinafter, and is representative of all three. As described in the '129 patent, the method basically comprises screening the raw carbide lime (size reduction), drying the screened carbide lime to obtain a low moisture content and a calcium carbonate content below 25 percent, fine grinding the dried material, and classifying the fine ground particles to separate a fraction of the particulates having a desired particle size, and corresponding lower specific gravity. As described in the '129 patent, this method results in a processed carbide lime composition which comprises 70 to 85 percent by weight calcium hydroxide and 5 to 25 percent by weight calcium carbonate, wherein the calcium carbonate is in the form of surface carbonation on the calcium hydroxide.
Since the advent of the processed carbide lime filler composition described in the '129 patent, certain benefits and drawbacks of the processed carbide lime composition have been discovered. One very significant beneficial quality discovered is that the processed carbide lime filler composition imparts sterilizing properties, unlike any other plastic or rubber filler or additive, to resin molded products in which the carbide lime filler composition is used as a filler material mixed with the resin. It is known that calcium hydroxide, a main constituent of the carbide lime filler composition, has antibacterial, anti-viral, and anti-fungal properties which can last up to a hundred years. Resin molded products made using the processed carbide lime filler composition apparently benefit from the calcium hydroxide content and are essentially sterilized. This characteristic makes resin molded products made with the processed carbide lime composition especially useful in applications where sterilization is important.
Another benefit imparted to polyvinyl chloride (PVC) resin molded products made with the processed carbide lime filler composition is the ability of calcium hydroxide to neutralize toxic gases from PVC combustion. Calcium hydroxide reacts favorably with toxic chloride gas produced by PVC combustion, resulting in two harmless substances, i.e., water and salt. A complementary quality is that the carbide lime filler composition is also flame retardant due to its high melting point, and exhibits a “hard char” effect after burning. Consequently, coating for electrical wiring, for example, which is made from a resin containing the processed carbide lime filler composition will not simply decompose when subjected to fire. Instead, the coating will form a hard coating on the wire, thus potentially preventing further hazard.
As explained in the '129 patent, commercial “synthetic” calcium hydroxide has been used, or the use thereof investigated, as fillers in resin molding systems. Some limitations and shortcomings of the use of calcium hydroxide as a filler are also described in that patent. It is also known to use calcium carbonate as a filler material in resin molding systems. There are numerous processes known for producing calcium carbonate, as described in, for example, U.S. Pat. Nos. 6,458,335 and 6,475,459 (“the '459 patent”). The '459 patent, for example, describes a process for producing precipitated calcium carbonate, and explains that the use of precipitated calcium carbonate is growing in various industries, such as paper, plastic, and pharmaceutical industries. The process for preparing calcium carbonate particles described in the '459 patent comprises reacting a starting material containing calcium oxide with carbonate ions in the presence of water to produce calcium carbonate, and recovering the calcium carbonate, characterized in that the formation is carried out directly from calcium oxide to calcium carbonate without intermediate stages. Additional, the process is carried out under intensive agitation such that the calcium carbonate becomes detached from the surface of the calcium hydroxide. Other methods known in the art for the manufacture of PCC are described in numerous patent applications listed in the '459 patent.
Calcium carbonate does not exhibit, and does not impart, the aforementioned anti-microbial or flame retardant properties to resin molded products. Unmodified calcium hydroxide used as a filler material in resin molding systems also does not impart the unique anti-microbial and flame retardant properties characteristic of the processed carbide lime composition. Unless calcium hydroxide is provided with the calcium carbonate surface carbonation, like the processed carbide lime composition, the desirable properties of the calcium hydroxide are not imparted to the resin molded product. This is apparently due to the surface carbonation providing a protective coating on the calcium hydroxide which permits the calcium hydroxide to be incorporated into the resin matrix in a manner in which unmodified calcium hydroxide cannot.
However, some disadvantages of the processed carbide lime composition made from raw “carbide lime,” and the method for making such composition, have also been discovered. One disadvantage is that, in the method described in the '129 patent for making the composition, controlling the process to provide the precise amount of surface carbonation is relatively complex. A disadvantage related to this problem pertains to the raw carbide lime material itself. In particular, the naturally occurring surface carbonation of the raw material varies greatly depending on the source of the raw carbide lime material, i.e., whether from a “wet” or a “dry” source. The effects of the surface carbonation are key to the composition exhibiting the desirable properties described above. Specifically, it is believed that a thin surface carbonation, of calcium carbonate, on a calcium hydroxide core enables the important properties of the hydroxide to be exhibited in the molded resin product. The thin coating of surface carbonation insulates the calcium hydroxide from reactions which occur in the resin molding process to enable the composition to be incorporated in a resin matrix. In the molded product, the calcium hydroxide eventually passes through the thin surface carbonation and spreads throughout the resin material, thereby imparting the aforesaid beneficial properties. If there is no, or insufficient, surface carbonation polymer degradation can occur due to contact with the high pH calcium hydroxide, in the presence of trace amounts of moisture through which hydroxide freely travels, in resins which are sensitive to high pH environments. If there is too much surface carbonation, the hydroxide core diminishes, and there is not exhibited the aforesaid advantageous properties. Accordingly, accurate control over the degree of surface carbonation is an important step in the process.
A significant disadvantage of the processed carbide lime filler composition produced from raw carbide lime is that the composition has a grayish color, or discoloration. Consequently, any resin molded product in which the composition is utilized as a filler can have only a gray, or darker, coloring. The process for producing acetylene gas involves the use of coke, and it is believed that the coke permanently stains the raw carbide lime waste by-product, resulting in the grayish discoloration of the resulting processed carbide lime composition. Commercially, this is an important limitation, since there are many applications in which white or light colored products are highly desirable. The grayish color thus detracts from the marketability of the composition produced according to the process in the '129 patent.
In view of the aforesaid disadvantages of the processed carbide lime filler composition and method of making the same described in the '129 patent, it would be desirable to produce a new carbide lime filler composition which has all of the desirable characteristics of the carbide lime filler composition made from the raw carbide lime material, but none of the disadvantages. Specifically, it would be desirable to produce a carbide lime filler composition having a white color, and to provide a method for making such white carbide lime filler composition in which control of the surface carbonation procedure is simple, accurate and consistently repeatable.